1. Field of the Invention
The present invention relates to a color picture tube, and more particularly, it relates to a color picture tube of a shadow mask type which is applicable to a display unit of high resolution employed in a terminal unit of a computer or the like.
2. Description of the Prior Art
FIG. 1 schematically illustrates the structure of a general color picture tube to which the present invention is applied.
Referring to FIG. 1, an envelope 1 of a glass vacuum vessel comprised by a front panel 2 whose inner surface is coated with a fluorescent screen 3 serving as a display surface, a funnel portion 4 connected with the front panel 2 and a neck portion 5 containing electron guns 6. A shadow mask 7 is suspended in the envelope 1 oppositely to the fluorescent screen 3 through pins (not shown) provided in a skirt portion of the front panel 2.
FIG. 2 illustrates relation between a circular-aperture type shadow mask having circular apertures and a display surface (screen). Symbol A denotes an enlarged part of the display surface 3 formed by a fluorescent screen and symbol B denotes an enlarged part of the shadow mask 7. Symbols X and Y denote a major axis and a minor axis of a color cathode-ray tube (CRT), i.e., an in-line array direction of electron guns 6 and a direction perpendicular thereto. The electron guns 6 are arrayed in order of BG for blue, GG for green and RG for red from the left-hand direction of FIG. 2.
Referring to FIG. 2, circular fluorescent dots 9 corresponding to three colors of blue, green and red for example are baked to the display surface 3 in correspondence to an aperture 8 of the shadow mask 7 as shown in the enlarged part B. Thus, the fluorescent display screen 3 as shown in the enlarged part A of FIG. 2 is formed by the fluorescent dots 9 of the three colors, while the fluorescent dots 9 of one of the three colors, e.g., those of red are in the pattern as shown at the enlarged part B. This also applies to those of the remaining two colors.
In general, the fluorescent dots 9 are larger in pitch than the apertures 8 of the shadow mask 7 for the reason that the shadow mask 7 is separated by a predetermined distance from the inner surface of the front panel 2 so that the aperture pattern of the shadow mask 7 is enlarged on the inner surface of the front panel 2. The ratio of such enlargement is generally about 4 to 5%, although the same depends on the size of the CRT and the structure of the electron guns 6.
One of structural disadvantages of the shadow mask type CRT is a moire phenomenon.
As is well known in the art, the moire phenomenon is observed in the form of fringes varied in density between two or more straight lines. In general, the moire phenomenon mainly appears as a moire pattern caused by optical interference of the interval between scanning lines and the array of the apertures 8 of the shadow mask 7.
FIG. 3 is a diagram for illustrating the array of the apertures 8 of the shadow mask 7 with reference to an aperture pattern of a partial region of the shadow mask 7 such as that in the vicinity of its center. Referring to FIG. 3, a line connecting apertures m.sub.3, m.sub.10 and m.sub.17 corresponds to the X axis of the display surface 3 while a line connecting apertures m.sub.8, m.sub.9, m.sub.10 and m.sub.11 corresponds to the Y axis of the display surface 3.
The aperture m.sub.10 corresponds to the central position of the shadow mask 7. This aperture m.sub.10 forms an equilateral triangle with the apertures m.sub.9 and m.sub.13 as well as another equilateral triangle with the apertures m.sub.13 and m.sub.14. Thus, the aperture pattern of the shadow mask 7 as shown in FIG. 3 is formed by an array of a plurality of equilateral triangles.
Description is now made on directions of adjacent apertures 8. For example, the aperture m.sub.10 is adjacent to the apertures m.sub.9, m.sub.13, m.sub.14, m.sub.11, m.sub.7 and m.sub.6. Thus, there are three directions of apertures adjacent to the aperture m.sub.10, i.e., the direction of the apertures m.sub.11, m.sub.10, m.sub.9 and m.sub.8 along the Y axis, that of the apertures m.sub.4, m.sub.7, m.sub.10, m.sub.13 and m.sub.16 at an angle of 60.degree. with respect to the Y axis and that of the apertures m.sub.2, m.sub.6, m.sub.10, m.sub.14 and m.sub.18 at an angle of -60.degree. with respect to the Y axis.
Seeing the array of the apertures in broad perspective, the series of m.sub.1, m.sub.8 and m.sub.15, the series of m.sub.5 and m.sub.12 and the series of m.sub.2, m.sub.9 and m.sub.16 appear linearly in parallel with the X axis on axes A.sub.0, A.sub.1 and A.sub.2 respectively. This also applies to those on other axes.
The moire pattern in question is mainly caused by optical interference between the pitch of the respective series of apertures on the axes A.sub.0, A.sub.1, . . . and the pitch of electron beams in scanning.
In the case of a CRT, the moire phenomenon takes place when two or more lines of different pitches are in parallel with and in specific relation to each other. For example, when M/N=m/n (m, n: positive integers) assuming that the scanning line pitch is M mm and the pitch of the linear series formed by the phosphor dots is N mm, it is preferable to avoid such relation that both of the integers m and n are any of one to four, in order to obtain a good result.
In other words, it runs as follows:
(1) It is preferable that interference fringes formed by the pitches M and N are of a small pitch.
(2) Difference in variable contrast on the screen is preferably small even if the interference fringes are of the same pitch.
These two points are requisites for solving the problem of the moire phenomenon. In the case of the CRT, it is important to reduce the variation of contrast (the degree of variation of the brightness) in order to reduce moire fringes, to put it strongly.
The shadow mask 7 is invisible in the exterior of the CRT, and hence the practical subject of discussion is the pitch of the fluorescent dots 9 forming the display surface 3.
Recently, a display unit for a terminal unit of a computer or the like has been improved with higher resolution while a display image on a display surface of a CRT is highly densified with thinner electron beams and a finer fluorescent dot pitch. As the result, a moire phenomenon caused by optical interference between the pitch of signals (picture signals) and the array of the apertures of the shadow mask has come into question.
With respect to generation of moire fringes, signals of trouble making are mainly vertical fringes. Referring to FIG. 3, the optical interference in question takes place between straight lines formed by the apertures 8 of the shadow mask 7, i.e., a train of the apertures m.sub.1, m.sub.2, m.sub.3 and m.sub.4 along an axis B.sub.0, a train of apertures m.sub.5, m.sub.6 and m.sub.7 along an axis B.sub.1 and those of respective apertures along axes B.sub.2, B.sub.3, B.sub.4, . . . and the pitch of signals (of linear image). Also in this case, the moire phenomenon can be explained through the relation of M/N=m/n similarly to the interference with the scanning line pitch.
Description is now made in further detail with reference to FIGS. 4(A) and 4(B).
FIG. 4(A) shows a pattern of, e.g., red fluorescent dots in a conventional fluorescent screen 3 or an array of apertures of a shadow mask 7. When vertical lines (straight lines along a direction Y) are displayed on the display surface (fluorescent screen) 3, symbol W denotes the width of signals, i.e., that of electron beams. Considering a section along the X axis of the display surface 3, the light emission state thereof is as shown in FIG. 4(B), in alignment with the dot array shown in FIG. 4(A). Although the optical outputs are discontinuous as obvious from FIG. 4(B), the same are continuously seen by the human eye, which recognizes objects macroscopically. From a reverse point of view, the display surface 3 is formed by a sufficiently small dot pitch applicable to resolution of the human eye.
There are two problems with respect to the moire phenomenon. One of the problems resides in the size b as shown in FIG. 4(B), which represents the dot pitch of the components in the direction X, i.e., the aperture pitch of the shadow mask 7, which causes a moire phenomenon similar to that through the aforementioned relation M/N=m/n. The other problem resides in relation a.noteq.b and b.noteq.c on ends of electron beams. Assuming that three beams of red, blue and green hit a completely identical position on the display surface 3, white luminescence is observed in the inner sides (central portions) of the electron beams since all of the three-color dots emit light, whereas the color balance is lost to provide colored luminescence at beam end portions due to the aforementioned relation of a.noteq.b and b.noteq.c as well as variation of dot positions depending on the colors of the fluorescence.